eBay is your best bet, try getting a faulty Xbox and ask for the error codes from the seller like I did, then you can search for them online and know if it's not related to the motherboard. Be specific and question them on why they are selling it and more importantly, has it ever been opened and if the security/warrenty sticker is still intact. That way it could just be a defective hardrive which is easy to replace and if you find that the seal was tampered with, PayPal will be on your side if you decide to return it for a full refund. I use my GoPro headcam when I open these kinds of boxes or basically anything expensive on auction sites as you can then use that as evidence.
After a while I managed to get a stripped down motherboard on its own that was tested and sold as spares from a working Xbox from a highly rated seller, if you're patient there's always one out there from time to time and will usually be cheaper. Make sure the revision or date of manufacture is similar to yours. IE, Scorpio edition, regular or came bundled like your if so. They change internals all the time so older consoles may have different screw holes and whatnot
Word of warning, if you need the bluray drive IE don't want to go completely digital I'm pretty sure each drive is paired to the motherboard during manufacture and is unique to another identical drive therefore you must get both from the same console.